Method to reduce birefringence and polarization mode dispersion in fiber gratings

ABSTRACT

Side-writing a refractive structure, such as a grating, into a waveguide results in an asymmetry in the induced refractive index change and a preferential orientation of dipolar defects, that leads to birefringence and polarization mode dispersion (PMD) in the refractive structure. Illumination of the structure to photo-reduce and to randomize UV-absorbing defects in the waveguide results in a reduction of the birefringence and the PMD.

FIELD OF THE INVENTION

[0001] The present invention is directed to optical waveguides, and moreparticularly to an approach for reducing the birefringence andpolarization mode dispersion in refractive structures, such as gratings,formed in optical waveguides.

BACKGROUND

[0002] Changes in the refractive index of photosensitive optical fiberscan be inscribed by exposing the fiber, usually the fiber core, but alsothe cladding in some circumstances, to UV radiation. Where theilluminating UV radiation has a periodic nature, the resultant change inthe fiber's refractive index is also periodic, resulting in theinscription of a grating. Examples of such gratings include fiber Bragggratings (FBGs) and long period gratings (LPGs).

[0003] The photosensitive fiber is often written with UV illuminationfrom the side of the fiber, in the direction perpendicular to the fiberaxis. Since the UV light is exponentially absorbed within thephotosensitive fiber, side-illumination results in a change in thefiber's refractive index that is non-uniform across the fiber. Further,the orientation of the dipole moments of the defects created by the UVillumination can increase depending on the polarization of the lightused to write such gratings. These two sources of asymmetry, thenon-uniform refractive index profile and the preferential dipoleorientation, lead to birefringence in the fiber, which may result indifferent spectral responses for orthogonal polarizations of waveguidedlight that is incident on the grating. This may also lead topolarization mode dispersion (PMD) in FBG's due to the different groupdelays (DGDs) for orthogonal polarizations. PMD is particularly aproblem for chirped gratings as it significantly affects the performanceof high-speed single-mode fiber optical communication network systems.

Summary of the Invention

[0004] There is a need to address the problems of UV-inducedbirefringence and PMD. In particular, there is a need to reduce theUV-induced birefringence and PMD of UV-inscribed refractive structuresin waveguides, for example FBGs and LPGs formed in optical fibers.

[0005] A Method in accordance with the present invention is directed toilluminating the UV exposed region of the waveguide with light,typically visible or infrared light, to reduce the UV-inducedbirefringence and the PMD of the UV-inscribed structures. In oneparticular embodiment, the invention is directed to a method forreducing the birefringent characteristics of a refractive structurewritten in a waveguide. The method includes providing the waveguide withthe refractive structure written in the waveguide, the refractivestructure having an associated germanium-related defect. The method alsoincludes exposing the refractive structure to photo-reducing lightabsorbable by the germanium-related defect and having a sufficientintensity so as to reduce a birefringent characteristic of therefractive structure.

[0006] Another embodiment of the invention is directed to an opticalwaveguide device formed from an optical waveguide. A refractivestructure is written in a portion of the waveguide containing aphotosensitive species. The refractive structure isbirefringence-reduced using photo-reducing light.

[0007] Another embodiment of the invention is directed to an opticalwaveguide device that includes an optical waveguide. A chirped Bragggrating in the waveguide has a bandwidth of at least 1 nm and apolarization mode dispersion of not more than 1 ps.

[0008] Another embodiment of the invention is directed to an opticalcommunications system that has an optical transmitter transmittingoutput light, and a fiber optic link coupled to carry the output lightfrom the optical transmitter. A waveguide having a grating is coupled tothe fiber optic link to reflect at least a portion of the output light.The grating includes a refractive structure written in a portion of thewaveguide containing a photosensitive species. The refractive structureis birefringence-reduced using photo-reducing light.

[0009] The above summary of the present invention is not intended todescribe each illustrated embodiment or every implementation of thepresent invention. The figures and the detailed description that followmore particularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention may be more completely understood in considerationof the following detailed description of various embodiments of theinvention in connection with the accompanying drawings, in which:

[0011]FIG. 1 schematically illustrates one approach to UV writing agrating in an optical fiber;

[0012]FIG. 2 schematically illustrates an apparatus useful for measuringbirefringent effects in an optical fiber;

[0013]FIG. 3 presents curves showing the transmission loss in a fiberbefore and after being exposed to photo-reduction according to anembodiment of the present invention;

[0014]FIG. 4 schematically illustrates an embodiment of an apparatusused for photo-reducing birefringent effects in an optical waveguide;

[0015]FIG. 5 presents curves showing transmission loss through a fiberfor several different durations of exposure to the photo-reducing light;

[0016]FIG. 6 presents a graph showing measured birefringence as afunction of exposure time;

[0017]FIG. 7 presents curves showing the transmission loss through thefiber after being exposed to photo-reducing light of differentpolarization states;

[0018]FIGS. 8A and 8C present topographic images of an optical fiber,obtained using an atomic force microscope, before and after exposure tothe photo-reducing light;

[0019]FIGS. 8B and 8D present cross-sectional measurements of an opticalfiber, obtained using an atomic force microscope, before and afterexposure to the photo-reducing light;

[0020]FIG. 9 presents curves showing PMD for a grating before and afterexposure to the photo-reducing light, using a first method of measuringPMD;

[0021]FIG. 10 presents a graph showing fractional reduction of PMD as afunction of exposure pulse energy;

[0022]FIG. 11 presents curves showing PMD for a grating before and afterexposure to the photo-reducing light, using a second method of measuringPMD;

[0023]FIG. 12 presents a graph showing reduction of PMD as a function ofphoto-reduction time;

[0024]FIG. 13 presents curves showing the change in absorption spectrumof the chirped fiber grating before and after exposure to thephoto-reducing light; and

[0025]FIG. 14 schematically illustrates an embodiment of acommunications system that includes a fiber grating according to thepresent invention.

[0026] While the invention is amenable to various modifications andalternative forms, specifics thereof have been shown by way of examplein the drawings and will be described in detail. It should beunderstood, however, that the intention is not to limit the invention tothe particular embodiments described. On the contrary, the intention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

[0027] The present invention is applicable to optical devices writteninto optical waveguides, such as optical fibers.

[0028] Refractive structures, for example gratings, are often inscribedin optical waveguides, such as optical fibers or planar waveguides,using UV light incident from the side of the waveguide. This method ofillumination allows the periodicity of the structure written in thewaveguide to be set at a selected value. There are, however, certaindisadvantages associated with this method of writing the structure.First, since the UV light is exponentially absorbed across thewaveguide, transverse illumination results in a change in thewaveguide's refractive index that is non-uniform across the waveguide.Further, the orientation of the dipole moments of the defects created bythe UV illumination can increase depending on the polarization of thelight used to write such gratings. These two effects predominantly leadto an increase in the birefringence of the fiber grating, which is oftenundesirable. For example, the induced birefringence leads to differentspectral responses for guided light in different polarization statesthat is incident on the structure. This may also lead to polarizationmode dispersion (PMD) due to the different group delays for orthogonalpolarizations.

[0029] The present invention is directed to the use of light, typicallyvisible or infrared light, to illuminate the UV-inscribed region of thewaveguide. It is believed that the light photo-reduces and randomizesthe Ge-1 defects that result from UV-inscription of a refractivestructure, thus reducing the birefringence and otherbirefringence-related characteristics. The term photo-reduction is usedto refer to the process described herein where light, for example lightin the visible and infra-red regions, is used to reduce the increasedabsorption, birefringence and PMD of a fiber grating due to asymmetricalexposure of the waveguide to UV light when the grating is written.

[0030] In several of the illustrative examples discussed below, thetechnique is directed to reducing the birefringence and PMD of a chirpedfiber grating (CFG). The selection of illustrative examples is notintended to imply any limitations to the use of the technique, and itwill be appreciated that the invention may be used for other types ofgrating, or indeed any other type of UV-inscribed structure, in a fiber.Furthermore, the invention is not limited to use in only fiberwaveguides, but may also be used in other types of waveguides, forexample planar waveguides.

[0031] One particular embodiment of a system 100 for UV-inscribing agrating structure into a fiber waveguide is schematically illustrated inFIG. 1. The system includes a UV light source (UVLS) 102 that generatesa conditioned light beam 104 at a wavelength suitable for exposing apattern in the photosensitive fiber 106. The UVLS 102 may include, forexample, an ultraviolet laser such as a Kr*F excimer laser or afrequency doubled Ar⁺ laser, along with optical systems for uniformizingthe beam intensity profile. The conditioned beam 104 is incident on aphase mask 108, which transmits a zero-order beam 110 and diffracts aminus first order beam 112. The two beams 110 and 112 interfere, causingan interference pattern 114 in the photosensitive fiber 106. In theparticular embodiment illustrated, the fiber 106 has a photosensitivecore 116. It will be appreciated that the fiber 106 may have aphotosensitive core and/or a photosensitive cladding. Exposure of thephotosensitive core 116 to the interference pattern 114 results inwriting a refractive index grating in the core 116. The light 104 istypically line focused by a cylindrical lens 118 onto the fiber 106.

[0032] The phase mask 108 includes a diffracting structure that sets theangle, θ, between the two beams 112 and 114 to have a selected value, soas to set the periodicity of the resulting fiber grating written in thefiber 106.

[0033] It will be appreciated that other approaches may be used to writea fiber grating. For example, an interference pattern may be created byfirst splitting a UV light beam into two beams using a beamsplitter andthen overlapping the resulting two beams so as to interfere in the fibercore. Additional approaches that may be used for writing refractiveindex structures into waveguides, particularly fibers, are described inU.S. Pat. Nos. 5,912,999, 6,035,083 and 6,404,956, incorporated hereinby reference.

[0034] One particular method useful for determining birefringent effectsin an optical waveguide that arise from side-writing a refractivestructure is the so-called “fixed analyzer method,” schematicallyillustrated in FIG. 2. Light 202 from a broadband light source (BLS) 204is passed through a polarization control unit 206, for example apolarizer, a retardation waveplate, or a combination of both. Thepolarization control unit 206 is used to select a particularpolarization state for the light 207 that enters the fiber 210. Thelight is then focused into the optical fiber 210 under test using a lenssystem 208 having one or more lenses. The fiber 210 includes aUV-inscribed structure 212, such as a grating. The broadband lightsource 204, for example a white light source AQ-4303 B from AndoElectric Co., Ltd. Tokyo, Japan, typically emits over a large bandwidth,for example 0.4 μm-1.8 μm.

[0035] The light 214 output from the fiber 210 may first be collected bya condensing lens system 216 and then passed through a polarizationanalyzer 218. The polarized light 220 is then passed into a detectorsystem 222 that detects the throughput from the analyzer 218 as afunction of light wavelength. The detection system 222 may be, forexample, an optical spectrum analyzer (OSA), such as a model AQ-6315Afrom Ando Electric Co., Ltd. Tokyo, Japan, having a measurementwavelength range from 0.35 μm-1.75 μm. The relative orientation of thetransmission axis of the analyzer 218 and the polarization direction ofthe light 207 may be set to any angle so as to maximize the contrast ofthe spectral fringes recorded at the output.

[0036] When the fiber 210 is birefringent, predominantly due to theUV-inscribed structure 212, a linearly polarized light launched, forexample at 45° to the birefringence axis, is split into orthogonalcomponents upon transmission. Upon exiting the fiber 210, thepolarization analyzer 218 orientated either parallel or perpendicular tothe polarizer 206 allows the superposition of the two orthogonalelectric field components in equal magnitude as propagated in the fiber210. This results in high-contrast fringes due to interference in thespectral domain. From the measured interference spectra thebirefringence can be calculated using Δn=λ²/(dλ L), where, d) is theperiod of the interference fringe, λ is the wavelength and L is thelength of the UV-exposed fiber.

EXAMPLE 1

[0037] The intrinsic birefringence of a 50 m length of pristine 3M TF19photosensitive fiber was measured to be approximately 1.16×10⁻⁷. A CFGhaving a length of 1.24 m was UV-inscribed in the fiber by side-writingwith a UV light beam. The resulting transmission spectrum, obtainedusing a system as illustrated in FIG. 2, is shown as curve 302 in FIG.3. The spectrum measured prior to photo-reduction, curve 302, showstransmission peaks at around 1100 m and 1350 m-n, suggesting a fringeperiodicity of about 250 nm. This corresponds to a value ofbirefringence of approximately 4.37×10⁻⁶ across the measured wavelengthrange.

[0038] An example of a photo-reduction system 400 that may be used forphoto-reducing the UV-induced birefringence of the fiber isschematically illustrated in FIG. 4. The system 400 includes a lightsource 402, typically a laser, and a focusing lens system 404. In theillustrated embodiment, the focusing system 404 focuses the light 406from the high intensity light source (HILS) 402 into the fiber 408, sothat the light 406 coupled into the fiber 408 is guided along the fibercore over the length of the fiber 408 at relatively high intensity. Theintensity or the energy of the light guided through the fiber core ismeasured using a detector 410 at the output of the fiber 409.

[0039] In another approach, the light 406 is not guided by the fiber408, but is incident on the fiber 408 from the side.

[0040] The light source 402 may be, for example, any suitable laser thatgenerates high average power or high pulse energy. For example, thelight source 402 may be a Q-switched Nd:YAG laser, or a pulsed opticalparametric oscillator, such as a Nd:YAG-pumped XPO laser (Infinity-XPOlaser, Coherent, Inc. CA) laser or a cw ion laser.

[0041] A polarization control element 412 may be positioned between thelight source 402 and the fiber 408. The polarization control element 412may include, for example, a waveplate to rotate the direction ofpolarized light produced by the light source, or to change thepolarization state from linear to elliptical or circular, or vice versa.The polarization controller 412 may also include a depolarizer toscramble the polarization of the light received from the light source402 or may include a polarizer to polarize unpolarized light receivedfrom the light source 402.

[0042] The 3M TF19 photosensitive fiber used to generate the data forcurve 302 in FIG. 3 was exposed to high-intensity, linearly polarizedlight. The optical intensity in the fiber was calculated to be a fewMW/cm², using the value of the energy of the light guided through thecore and the core diameter. The light source was a Q-switched Nd:YAGpumped XPO laser (Infinity-XPO laser, Coherent, Inc. CA) operating at 10Hz, and generating 5 ns pulses at a wavelength of 532 nm. The light wasfocused axially into the core of the fiber, and the fiber was exposed tothe XPO-laser light for about 15 minutes.

[0043] The spectrum measured after the exposure, curve 304, shows analmost complete erasure of the UV-induced birefringence. Thecharacteristics of the grating, which are manifested as a loss peakcentered at around 1550 nm, were substantially preserved during thephoto-reduction process. The erasure of UV-induced birefringence wasfurther verified by rotating the polarizer 206 and analyzer 218 anglesfor any possible rotation in the birefringence axis of the fiber 210during exposure to polarized visible radiation.

[0044] It is believed that the birefringence that results fromUV-writing a refractive structure in a waveguide arises predominantlyfrom the presence of Ge-1 defects, which has maximum absorption around280 nm with the absorption tail extending into the visible and nearinfrared wavelength regions. Illumination of the written waveguide withlight absorbable by this defect results in photo-reducing the absorptionband, with a concomitant reduction in the UV-induced absorption loss.

[0045] Light may be absorbed by the Ge-1 defect in a single photonabsorption process. Such light, however, lies in the UV region and isstrongly absorbed by the Ge present in the waveguide, and therefore isbetter suited to illuminating the waveguide from the side thanilluminating the waveguide by propagating along the core. Light having alonger wavelength may be absorbed by the defect via a multi-photonabsorption process. Since it is not absorbed as strongly by the Ge inthe waveguide, such light is well-suited to illuminating the waveguideby propagating along the waveguide core, so long as its wavelength isless than the cut-off wavelength of the waveguide. For example, thewavelengths used in the examples described here, 514.5 nm, 532 nm and650 nm, are absorbed via a two-photon absorption process. Otherwavelengths may also be used, for example in a three or four-photonabsorption process. Additionally, the waveguide may be illuminated bytwo or more wavelengths of light, with the Ge-1 defect absorbing one ormore photons at each of the different wavelengths.

EXAMPLE 2

[0046] Another experiment was carried out using a cw Ar⁺ ion laseroperating at λ=514.5 nm to illuminate the fiber core containing theinscribed grating. A fixed laser power of 500 mW was used in thisexample. The light produced by the laser was linearly polarized, and aquarter wave retardation plate was placed between the laser and thefiber to circularly polarize the light before entering the fiber core.The spectral fringes were measured as a function of time using a systemas illustrated in FIG. 2. The transmission fringe spectra thus measuredare illustrated as different curves in FIG. 5. Curve 502 represents thetransmission measured before the fiber, inscribed with a grating, wasilluminated with the Ar⁺ laser. Curves 504, 506 and 508 represent theloss measured after 1 hour, 2 hours and 3 hours respectively.

[0047] As the number of interference fringes is proportional to theUV-induced birefringence, it is clearly seen from FIG. 5 that the numberof fringes, and hence the birefringence decreases with exposure time fora fixed laser power of approximately 500 mW. The birefringence wascalculated as a function of exposure time, and is plotted as curve 602in FIG. 6. The calculated value of birefringence is shown, along withthe associated error, for each exposure time. The curve 602 representsthe best linear fit to the data points.

[0048] The PMD of the grating, calculated using (DλΔn/n_(eff)) where Dis the dispersion of the grating (650 ps/nm), was calculated for thefiber before being exposed and after being exposed for three hours.Before exposure, the PMD was 6.48 ps, which reduced to 3.26 ps after 3hours, showing a reduction of approximately a factor of two under theseillumination conditions.

EXAMPLE 3

[0049] This example describes an attempt to verify one of the mechanismsof photo-reduction of the UV-induced birefringence in the fiber—theeffect of dipole orientation of the UV-induced Ge-1 defects. One of themain sources of UV-induced birefringence is thought to be an anisotropythat arises when dipole moments are created when the 240-nm absorptionband is bleached with polarized UV light during the grating writingprocess. This anisotropy, due to preferential orientation of thedipoles, is expected to give rise to birefringence when it lies in aplane perpendicular to the light propagation direction and has little orno effect when it is parallel to the propagation direction of the guidedlight. The preferential orientation of the dipoles created duringexposure to polarized UV light can be randomized by either launching acircularly or depolarized polarized light through the fiber core. Thiseffectively reduces the birefringence of the fiber.

[0050] To demonstrate this first a linearly polarized light beam fromthe cw Ar⁺ laser having a power of 500 mW was launched into the fibercore. The 3M TF19 photosensitive fiber used had a CFG having a bandwidthof 14.7 nm and was written with a dispersion of 1600 ps/nm,corresponding to the UV-exposed fiber length of 2.44 m.

[0051] The birefringence spectrum was measured, curve 702 in FIG. 7,from which the birefringence was calculated to be approximately 6×10⁻⁶.The fiber was then exposed to circularly polarized light at a powerlevel of 500 mW in the fiber core, for duration of 120 mins. Thebirefringence spectrum was measured again, curve 704, from which thebirefringence was measured to be about 4.5×10-6. The level ofbirefringence could be reduced further either by increasing the exposuretime or the launch power of the laser.

[0052] The results illustrated in FIG. 7 suggest that the dipoleorientation of UV-induced defects acts as a possible mechanism forbirefringence in the UV-inscribed structures. Randomizing thepreferential dipole orientation by exposure to circularly polarizedlight significantly reduces the UV-induced birefringence.

EXAMPLE 4

[0053] This example discusses another possible mechanism responsible forthe UV-induced birefringence—geometrical asymmetry in the refractiveindex profile of the fiber core due to the UV exposure process.Cross-sections of the UV-exposed fibers are profiled using an atomicforce microscope (AFM) after cleaving the fiber in the exposed regionand etching the cleaved end face using dilute hydrofluoric acid (HFA)for a short time. The UV exposure results in differential etching at theside exposed to the UV light. Exposing the core of the UV-exposed fiberto photo-reducing visible radiation significantly reduces the UV-inducedasymmetry and hence reduces the birefringence.

[0054] To demonstrate the AFM process, a 3M TF19 photosensitive fiber,written with a grating, was measured before and after treatment with thephoto-reducing radiation. A pulsed Nd:YAG laser operating at 532 nm witha core-launched pulse energy of 30 mJ was used to photo-reduce thedefects in approximately 30 minutes. Both the UV-exposed andphoto-reduced fibers were then cleaved and etched in fresh 48% HFA forapproximately 5 seconds. The fibers were then rinsed with de-ionizedwater and dried using a compressed inert gas. The samples were thenmounted in custom fabricated mini-vices and placed in the AFM forimaging. The AFM (Digital Instruments Dimension 5000 SPM) was used inthe tapping mode. The probes used were Olympus OTESPA Silicon with aforce constant of nominally 40 N/m and a radius of curvature <10 nm.

[0055] The AFM images of the fibers are shown in FIGS. 8A and 8C beforeand after the photo-reducing exposure respectively. FIGS. 8B and 8Dpresent cross-sectional data for a slice across the fiber, before andafter the photo-reducing exposure, respectively. It can be seen fromthese figures that the asymmetry in the fiber cross-section due to theUV exposure from one side is significantly reduced after exposure to thephoto-reducing radiation. As mentioned before, this reduces thebirefringence arising due to the geometrical asymmetry.

EXAMPLE 5

[0056] The first-order polarization mode dispersion (PMD) of a chirpedfiber grating is determined by the local birefringence of the fiber andthe polarization mode coupling. The local birefringence of a fiber,however, is dominated by the UV-induced birefringence that arises fromthe grating writing process. The intrinsic birefringence of the fibergrating, before the grating is written, is about 10-7.

[0057] For a direct measurement of the reduction in the PMD due to thephoto-reduction process, 18 identical chirped Bragg gratings (D 420ps/nm, AS=1.5 nm and L is approximately 7 cm) were written using asystem as disclosed in U.S. Pat. Nos. 5,912,999, 6,035,083 and6,404,956. The bandwidth, A?, of a grating is defined as the range ofwavelengths where the grating reflectivity is within 1 dB of the maximumgrating reflectivity value.

[0058] The gratings were annealed at 120° C. to out-diffuse D₂ and tostabilize the index change. Chirped Bragg gratings are useful forcompensating the dispersion on an optical signal that has propagatedalong a long length of optical fiber. Under normal dispersion, thelonger wavelengths of the optical signal propagate faster than theshorter wavelengths of the signal, and so the pulse is stretched out,with longer wavelengths reaching the chirped Bragg grating earlier thanthe shorter wavelengths. The chirped Bragg grating is designed so thatthe longer wavelengths have to propagate further into the grating beforebeing reflected, whereas the shorter wavelengths are reflected at thefirst portion of the grating. Accordingly, the shorter wavelengthspropagate over a shorter path before being reflected than the longerwavelengths. The longer and shorter wavelength components of the signalare overlapped in the reflected pulse, and the pulse is compressedrelative to the incoming pulse.

[0059] The grating characteristics were measured using an optical vectoranalyzer (OVA) 1550 (LUNA Technologies Inc., Blacksburg, Va., USA). TheOVA uses interferometry to measure the necessary complex amplitudes andconstruct the Jones matrix, which contains all the parameters such asinsertion loss, group delay, polarization dependent loss andpolarization mode dispersion that characterize the device. The values ofPMD for the gratings were verified by independent measurements madeusing a 4-point modulation phase-shift (MPS) technique.

[0060] Different exposure conditions were selected to study the effectof pulse energy and exposure time on the reduction in PMD due to thephoto-reduction effect. A set of 3 gratings was used for each exposurecondition in the study. Table I shows a summary of the differentexposure conditions tested. The gratings were exposed to pulses of lightat 532 nm coupled into the core from a Q-switched Nd:YAG pumped XPOlaser (Infinity-XPO laser, Coherent, Inc. CA). The different exposureconditions represented different combinations of pulse energy coupledinto the fiber core and the corresponding exposure time. TABLE I Summaryof exposure conditions Pulse energy Exposure time 1 mJ 5 mJ 10 mJ 30 mJ 5 min. ✓ ✓ 10 min. ✓ 30 min. ✓ ✓ 60 min. ✓

[0061] Grating #2 was exposed to 30 mJ pulses for duration of 60minutes. The PMD measured using the OVA prior to exposure is shown ascurve 902 in FIG. 9, within the bandwidth of the grating. Themeasurement was carried out in the high-resolution mode with 1.35 pmresolution bandwidth and then a 160 pm filter was applied to get thebehavior of the first order PMD value of the device. The PMD valuemeasured after the exposure to the photo-reducing light is shown ascurve 904. The first-order PMD of the grating decreased fromapproximately Ips to approximately 0.12 ps, a reduction of approximately90%, after the exposure. The PMD of gratings, which were not exposed,remained the same at approximately 1 ps.

[0062] Thus, the technique is useful for producing a PMD value of lessthan 1 ps in a chirped fiber grating having a bandwidth of at least 1 nmand/or a reflectivity in excess of 50%. The technique may also be usedto produce a PMD value of less than 0.75 ps, less than 0.5 ps and lessthan 0.25 ps in a fiber grating having a bandwidth of at least 1 nm, asis shown below. In addition, the technique may be used for reducing thePMD where the peak reflectivity of the grating is at least 80%, and insome cases at least 90%.

[0063] Using the PMD measurements made before and after thephoto-reduction exposure (as mentioned in Table I), one may calculatethe percentage PMD reduction for the different conditions. FIG. 10 showsa curve relating the percentage of reduction in PMD to the pulse energy.Each data point is an average for 3 gratings, exposed under identicalconditions. From the behavior illustrated in FIG. 10, it is clear thatthe greater the photon flux in the fiber core, the better is thephoto-reduction process, suggesting a multi-photon mechanism.

[0064] The measurements of PMD were confirmed by repeating the procedurefor a different grating and also by measuring the PMD of the gratingusing the 4-point MPS method using smaller step sizes of 16 pm and 1 pm.The results of the four-point MPS method are presented in FIG. 11. Curve1102 shows the PMD measured before exposure and curve 1104 shows the PMDmeasured after the exposure. A dramatic reduction in the PMD value ofthe grating is evident from these measurements also. In summary, it hasbeen shown that the illumination of a waveguide, which includes aUV-written refractive structure, with light that can be absorbed by theGe-1 defect leads to a reduction in the birefringent characteristics ofthe waveguide. In particular, the birefringence of the structure and thePMD of the structure are reduced. It is believed that the reduction inbirefringent characteristics occurs with photo-reducing of the Ge-1absorption.

[0065] The waveguide may be illuminated from the side or along its core.Illumination along the core leads to more efficient photo-bleaching ofthe defect. The absorption of the photo-reducing light may be singlephoton or multi-photon, and a wide variety of wavelengths may be used.The reduction of the birefringent characteristics varies approximatelylinearly with duration of exposure and energy of the exposing light.

[0066] The PMD of chirped Bragg gratings was also measured before andafter the photo-reducing exposure for fixed pulse energy as a functionof time. FIG. 12 is a plot showing the reduction in the PMD of CFG,measured using OVA 1500 (LUNA Technologies Inc., VA, USA). The bandwidthof the CFG was 1.5 nm with 420 ps/nm dispersion, corresponding to aUV-exposed fiber length of approximately 7 cm. The PMD of the CFG beforeexposure was approximately 1 ps, which after exposure to 532 nm laserlight with 40 mJ of pulse energy reduces to 0.3 ps in 15 minutes and to0.26 ps and 0.2 ps upon continuing the exposure for 30 minutes and 60minutes, respectively. Comparing the results with previous measurementsfurther confirms that increasing the pulse energy of light launched intothe fiber speeds-up the photo-reduction, suggesting a multi-photonprocess.

[0067] The waveguides treated according to the present invention aredifferent from those treated in some other manner to reduce or avoid thebirefringent effects that arise when UV-writing a refractive structure.One of the other approaches used for reducing birefringent effectsincludes illuminating the waveguide from different directions in orderto reduce the asymmetry that arises from UV-inscribing the structure byilluminating from one side. This requires careful registration of thewaveguide to the UV-writing pattern when rotating the waveguide to a newillumination position. Furthermore, this approach does not avoid orreduce the UV-absorption that arises from the Ge-1 defect: it simplymakes the presence of the defect more uniform.

[0068] Another approach to reduce the birefringent effects that arisewhen UV-writing a refractive structure includes altering thepolarization of the UV-inscribing beam. This is understood to align thedipoles associated with the defect in a manner that reducesbirefringence. This approach does not avoid or reduce the UV-absorptionthat arises from the Ge-1 defect.

[0069] In contrast with other approaches, the present approach not onlyreduces birefringent effects, but also reduces the UV-absorptionassociated with the Ge-1 defect in the waveguide. The UV-absorption inthe waveguide may be measured spectroscopically. Both the magnitude ofthe UV-absorption and the shape of the UV-absorption are affected by thephoto-reducing process.

[0070] As has already been mentioned, the absorption due to the presenceof Ge-1 defects in the waveguide exposed to UV light has a maximum atapproximately 280 nm and the absorption tail extends into the visibleand near infrared (NIR) wavelength regions. Changes in the absorptioncharacteristics, equivalently the attenuation (in dB/m), in the visibleand NIR wavelength regions can be monitored using a cutback measurement.The measurement requires launching light from a broadband light source(λ is approximately 0.4 μm-1.8 μm) (AQ-4303 B., Ando Electric Co., Ltd.Tokyo, Japan) through the fiber and measuring the changes over distanceusing an optical spectrum analyzer (AQ 6315-A, Ando Electric Co., Ltd.Tokyo, Japan). The method known as the cutback method involves couplingthe fiber to the source and measuring the power out of the far end. Thefiber is then cut near the light source and the power measured again. Byknowing the power at the source (P_(S)) and at the end (P_(E)) of thefiber and the length of the fiber (L), the attenuation coefficient (indB/m) can be determined by calculating [(P_(E)—P_(S))/L].

[0071] One example of the change in the fiber absorption that resultsfrom exposure to the photo-reducing light is presented in FIG. 13. Curve1302 shows the absorption spectrum before exposure and curve 1304 showsthe UV absorption after exposure to the photo-reducing light. To achievethe reduction in the absorption features of the fiber, the fiber wasexposed to circularly polarized 514.5 nm light from Ar⁺ laser with anaverage power of 500 mW launched in the core of the fiber forapproximately 2 hours. As can be seen, the photo-reducing processresults in significant-reduction of the absorption tail in the visibleand NIR wavelength regions. Another important result is that theabsorption at the signal wavelength, typically in the range 1500 nm-1600nm for optical communications, is reduced by about 0.1 dB/m comparedwith the grating-written fiber before photo-reduction.

[0072] Another approach useful for measuring the presence of thebirefringent defects is to perform an atomic force microscopy (AFM)measurement across the waveguide. This technique was discussed abovewith respect to Example 4.

[0073] Waveguide devices according to the present invention may be usedin many different types of systems. A low PMD fiber grating isparticularly useful in an optical communications system, for example ina dispersion compensating grating. One embodiment of an opticalcommunications system 1400 that includes a dispersion-compensatinggrating is schematically illustrated in FIG. 14. The system 1400 has anoptical transmitter 1402 that may include light sources 1404 a, 1404 b,. . . , 1404 m operating at different wavelengths λ1, λ2, . . . , λm,corresponding to different optical channels. The light sources 1404a-1404 m are typically externally modulated, single frequency lasersoperating at about 1550 nm, although they may be internally modulatedand may also operate at different wavelengths. The light from the lightsources 1404 a-1404 m is combined in an optical combiner unit 1406,which may include one or more wavelength division multiplexing (WDM)elements. The combined signal is launched into a fiber optic link 1408.

[0074] The fiber link 1408 may include one or more fiber amplifier units1410, for example rare earth-doped fiber amplifiers, Raman fiberamplifiers or a combination of rare earth-doped and Raman fiberamplifiers. The pump light may be introduced to the fiber amplifier 1410from a pump unit 1412 via a coupler 1414. Optical isolators (not shown)may be positioned along the fiber link 1408 to prevent light frompassing in the backwards direction. For example isolators may bepositioned on either side of the amplifier 1410 to reduce thepossibility of backscattered light, propagating towards the transmitter1402, or from being amplified in the amplifier 1410.

[0075] The fiber optic link 1408 may be coupled to a receiver 1420, forexample a multichannel receiver. Multichannel light enters the receiver1420 and is split up into different channels corresponding to differentwavelengths. In the illustrated embodiment, the receiver 1420 includesdetectors 1422 a, 1422 b, . . . , 1422 m corresponding to differentoptical channels at wavelengths λ1, λ2, . . . , λm. A wavelengthsplitting unit 1424, for example using a wavelength divisiondemultiplexing elements, may be used to split the incoming light intothe different wavelength components, and to direct the separated lightcomponents to their respective detectors.

[0076] In the illustrated embodiment, a dispersion compensating fibergrating 1426 is disposed to compensate for the dispersion of the lightpropagating along the fiber link 1408 from the transmitter 1402 to thereceiver 1420. The light propagating along the fiber link 1408 isdirected to the dispersion compensating fiber grating 1426 via acirculator 1428, which directs the light reflected by the grating 1426to the receiver 1420. Dispersion compensation may be provided at anypoint along the fiber link 1408, and is not restricted to being providedjust before the detector.

[0077] It will be appreciated that other arrangements of fiber gratingsmay be used for add/drop multiplexers, band-pass filters, singlefrequency reflectors, and the like.

[0078] As noted above, the present invention is applicable to techniquesfor reducing birefringence and PMD in UV-inscribed structures inwaveguides, and is believed to be particularly useful for reducing PMDin FBGs. The present invention should not be considered limited to theparticular examples described above, but rather should be understood tocover all aspects of the invention as fairly set out in the attachedclaims. Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

What is claimed is:
 1. A method for reducing birefringentcharacteristics of a refractive structure written in a waveguide,comprising: providing the waveguide with the refractive structurewritten in the waveguide, the refractive structure having an associatedgermanium-related defect; and exposing the refractive structure tophoto-reducing light absorbable by the germanium-related defect so as toreduce a birefringent characteristic of the refractive structure.
 2. Amethod as recited in claim 1, wherein providing the waveguide includesside-illuminating the waveguide with UV light at a first wavelength towrite the refractive structure in the waveguide, the refractive indexstructure having a non-uniform refractive index characteristic acrossthe waveguide.
 3. A method as recited in claim 2, wherein thephoto-reducing light has a wavelength different from the firstwavelength.
 4. A method as recited in claim 1, wherein exposing therefractive structure with the photo-reducing light results inrandomizing directions of dipoles of defects that arise from writing therefractive structure.
 5. A method as recited in claim 1, whereinexposing the refractive structure with the photo-reducing light resultsin a reduction in light absorption in the waveguide for ultraviolet,visible and infrared wavelengths.
 6. A method as recited in claim 1,wherein the refractive structure is a grating in the waveguide.
 7. Amethod as recited in claim 6, wherein the grating is a fiber Bragggrating.
 8. A method as recited in claim 7, wherein the fiber Bragggrating is a chirped fiber grating.
 9. A method as recited in claim 7,wherein the chirped fiber grating is a dispersion-compensation grating.10. A method as recited in claim 7, wherein the fiber Bragg grating isan unchirped fiber grating.
 11. A method as recited in claim 6, whereinthe grating is a long period fiber grating.
 12. A method as recited inclaim 1, wherein the waveguide is an optical fiber.
 13. A method asrecited in claim 1, wherein exposing the refractive structure includesguiding the photo-reducing light along the waveguide.
 14. A method asrecited in claim 1, wherein the photo-reducing light has a wavelengthsuch that the germanium-related defect undergoes a two-photon absorptionof the photo-reducing light.
 15. A method as recited in claim 1, whereinthe photo-reducing light has a wavelength such that the germanium-baseddefect undergoes a multiple photon absorption of the photo-reducinglight.
 16. A method as recited in claim 1, wherein the exposing isperformed until a desired value of the birefringent characteristic isachieved.
 17. A method as recited in claim 1, further comprising settingthe photo-reducing light in a desired polarization state before exposingthe refractive structure to the photo-reducing light.
 18. A method asrecited in claim 17, wherein the desired polarization state iscircularly polarized.
 19. A method as recited in claim 17, wherein thedesired polarization state is depolarized.
 20. A method as recited inclaim 1, wherein exposing the refractive structure to the photo-reducinglight includes exposing the refractive structure to pulsed laser light.21. A method as recited in claim 1, wherein exposing the refractivestructure to the photo-reducing light includes exposing the refractivestructure to continuous laser light.
 22. An optical waveguide device,comprising: an optical waveguide; and a refractive structure written ina portion of the waveguide containing a photosensitive species, therefractive structure being birefringence-reduced using photo-reducinglight.
 23. A device as recited in claim 22, wherein the opticalwaveguide is an optical fiber.
 24. A device as recited in claim 23,wherein the optical fiber is a single mode optical fiber.
 25. A deviceas recited in claim 22, wherein the refractive structure is a longperiod grating.
 26. A device as recited in claim 22, wherein therefractive index is a Bragg grating.
 27. A device as recited in claim26, wherein the Bragg grating is a chirped Bragg grating.
 28. A deviceas recited in claim 27, wherein the Bragg grating has a polarizationmode dispersion of no more than 1 ps.
 29. A device as recited in claim27, wherein the chirped Bragg grating is a linearly chirped Bragggrating.
 30. A device as recited in claim 27, wherein the chirped Bragggrating is a nonlinearly chirped Bragg grating.
 31. A device as recitedin claim 27, wherein the chirped Bragg grating is adispersion-compensation grating.
 32. An optical waveguide device,comprising: an optical waveguide; and a chirped Bragg grating in thewaveguide having a polarization mode dispersion of not more than 1 psand at least one of a bandwidth of at least 1 nm and a maximumreflectivity of at least 50%.
 33. A device as recited in claim 32,wherein the maximum reflectivity is at least 80%.
 34. A device asrecited in claim 32, wherein the maximum reflectivity is at least 90%.35. A device as recited in claim 32, wherein the polarization modedispersion is no more than 0.75 ps.
 36. A device as recited in claim 32,wherein the polarization mode dispersion is no more than 0.5 ps.
 37. Adevice as recited in claim 32, wherein the polarization mode dispersionis no more than 0.25 ps.
 38. A device as recited in claim 32, whereinthe maximum reflectivity is at least 80%.
 39. A device as recited inclaim 32, wherein the maximum reflectivity is at least 90%.
 40. A deviceas recited in claim 32, wherein the polarization mode dispersion is nomore than 0.75 ps.
 41. A device as recited in claim 32, wherein thepolarization mode dispersion is no more than 0.5 ps.
 42. A device asrecited in claim 32, wherein the polarization mode dispersion is no morethan 0.25 ps.
 43. A device as recited in claim 32, wherein the opticalwaveguide is an optical fiber.
 44. A device as recited in claim 43,wherein the optical fiber is a single mode optical fiber.
 45. A deviceas recited in claim 32, wherein the chirped Bragg grating has a linearchirp.
 46. A device as recited in claim 32, wherein the chirped Bragggrating has a nonlinear chirp.
 47. An optical communications system,comprising: an optical transmitter transmitting output light; a fiberoptic link coupled to carry the output light from the opticaltransmitter; and a waveguide grating coupled to the fiber optic link toreflect at least a portion of the output light, the waveguide gratingincluding a refractive structure written in a portion of the waveguidecontaining a photosensitive species, the refractive structure beingbirefringence-reduced using photo-reducing light.
 48. A system asrecited in claim 47, wherein the optical transmitter includes multiplelight sources generating light at multiple wavelengths and also includeslight combining units to combine the light at the multiple wavelengths.49. A system as recited in claim 47, further comprising an opticalreceiver unit to receive at least a portion of the output light.
 50. Asystem as recited in claim 49, wherein the optical receiver includesdetectors for detecting light at different wavelengths, and alsoincludes demultiplexing units for separating light signals at thedifferent wavelengths.
 51. A system as recited in claim 47, furthercomprising one or more optical amplifier units disposed along the fiberoptic link.
 52. A system as recited in claim 47, wherein the waveguidegrating is a chirped fiber grating.
 53. A system as recited in claim 52,further comprising a circulator unit, the chirped fiber grating beingcoupled to the circulator unit to receive light from the opticaltransmitter.